Photovoltaic devices provide a non-polluting, silent and reliable source of electrical power. Originally, photovoltaic devices were fabricated from crystalline materials and, as a consequence, were expensive and restricted in size. Techniques have now been developed for the preparation of large area, thin film semiconductor materials which may be advantageously fabricated into low cost, large are a photovoltaic devices.
It is frequently desirable to subdivide large area photovoltaic devices into a plurality of electrically interconnected small area devices disposed upon a common substrate. The structure of these arrays makes them more tolerant of defects in the photovoltaic material and allows for the selection of desired voltage and/or current outputs. In some instances, the small area devices of an array are interconnected in a series arrangement so as to provide for an increased voltage. In other instances, particularly for low voltage, high current applications such as electrochemical processes, a parallel-connected array is desired. A parallel-connected array provides a constant voltage which is independent of device area. While a single, large area body of photovoltaic material will provide a high current at a low voltage, it is still desirable to configure the material into an array of interconnected regions. Configuring a large area of body of photovoltaic material as a parallel-connected array allows for localization of defective regions in the photovoltaic material so that such defects may be readily removed without affect upon the remainder of the device.
Photovoltaic devices, and particularly thin film photovoltaic devices usually include a transparent, electrically conductive top electrode for collecting photogenerated current therefrom. The transparent, electrically conductive electrode material is generally of modest electrical conductivity, and large area devices must include current carrying structures such as grid lines, bus bars and the like for decreasing the series resistance thereof. These structures are fabricated from opaque materials and their presence upon the light incident surface of the photovoltaic device represents a loss of active area. Therefore, the prior art necessitated a balance between maximizing the active area of a photovoltaic device and minimizing its series resistivity. Furthermore, subdividing large area material into a plurality of devices involves scribing away portions of the semiconductor material, and the scribed regions also represent photovoltaically inactive areas. Clearly, it would be desirable to maximize the active area of photovoltaic devices by limiting the size of these inactive areas, while maintaining low series resistivity.
Laser scribing techniques are often employed in the fabrication of large area photovoltaic arrays since a laser can scribe precise, fine lines in the photovoltaic material at a fairly low cost, thereby minimizing the expense of device fabrication and maximizing active area. Problems have been encountered in connection with many prior art laser scribing techniques, because of the high levels of localized heating produced by the laser beam. Such heating can cause metal electrodes to melt and short circuit the device. Also, the laser can induce unwanted crystallization of amorphous semiconductor materials thereby increasing their electrical conductivity and creating shunted current paths which degrade device efficiency. U.S. Pat. No. 5,268,037, referred to hereinabove describes a parallel-connected array, preferably fabricated by a laser scribing process. The array is defined by a plurality of finely scribed lines which subdivide a large area body of photovoltaic material into a series of discrete photovoltaic devices electrically connected in parallel. While the method described therein provides efficient and reliable photovoltaic devices, it has been found that some problems can occur as a result of material being "thrown" during the laser ablation process used for forming the scribed lines. The thrown material can interfere with subsequent deposition steps and/or obscure portions of the photovoltaic device thereby reducing its active area. Also, the scribed lines themselves, while fairly narrow, do represent a loss of generating capacity which amounts to roughly 4% of the surface area of the device. Clearly, it would be desirable to minimize both the amount of thrown material and the inactive area.
The present invention provides an improved configuration of parallel-connected array in which subdivision and interconnection is accomplished by a series of spaced apart through-holes. Use of the through-hole connections effectively subdivides the photovoltaic material into separate photovoltaic regions and provides a shortened current path through the transparent conductive layer, thereby eliminating the need for any metallic, top surface mounted, current-collecting structures.
Through connections have previously been employed in connection with photovoltaic devices for a variety of purposes, and in a variety of configurations. U.S. Pat. No. 4,865,999 of Xi shows a photovoltaic device which includes a series of point contacts therethrough for establishing electrical communication between a current collecting grid and a transparent electrode. The point contacts are manufactured from a printed conductive ink, and are relatively large in diameter and widely spaced. U.S. Pat. No. 4,981,525 of Kiyama discloses a series-connected array of photovoltaic devices, in which each of the individual devices includes a number of through-hole connections for carrying current from the transparent conductive electrode to a subjacent current-collecting structure. The through-hole connections are fairly large and widely spaced. In most instances, the device is fabricated upon a transparent, electrically conductive substrate, and the through-hole connections establish communication between a top metallic electrode and the bottom transparent electrode. Several embodiments are shown wherein the device is fabricated upon a bottom, metallic electrode; however, this electrode is a relatively thin metallic body supported upon an electrically insulating substrate. It has been found that structures of this type are incompatible with high-speed laser processing, because of the impossibility of preventing damage to the thin metallic bottom layer.
It is to be appreciated that there is a need for parallel-connected monolithic arrays of photovoltaic devices which may be readily manufactured by high-speed laser patterning processes. It is further desirable that the structure minimize the amount and effects of thrown material and not require any undue caution in the fabrication thereof. As will be detailed hereinbelow, the present invention provides for a parallel-connected photovoltaic array which may be readily fabricated by a laser patterning process. The device makes efficient use of material and is easy to fabricate. These and other advantages of the present invention will be readily apparent from the drawings, discussion and description which follow.